neutron diffraction study of hydrogen thermoemission ......neutron diffraction method was chosen as...

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Neutron Diffraction Study of Hydrogen Thermoemission Phenomenon

from Powder Crystals

I. Khidirov Institute of Nuclear Physics, Uzbekistan Academy of Science

Uzbekistan

1. Introduction

In hydrogen energy powder metals or compounds that accumulate and desorb hydrogen extensively are used for hydrogen storages. Even in small quantities hydrogen can have stronger effect on structure and properties of some metals and compounds than heavier elements. Study of both hydrogen arrangement in crystal structure and process of its evacuation out of a lattice is of great importance for understanding the reasons and mechanisms of hydrogen influence on material properties. At vacuum hydrogen evacua-tion, it is possible to find such temperature Тev at which the hydrogen atoms, having low bond energy and high diffusion speed, are eliminated out of the lattice. The configuration of relatively heavy matrix atoms (previously stabilized by hydrogen atoms) does not change because of their insufficient diffusive mobility at this temperature. In this way it is possible to obtain new metastable phases artificially (hydrogen induced phases). In these phases the structure of initial hydrogenous phase can be kept (as if "frozen"), but during heat treatment they can undergo phase transitions uncharacteristic for the initial material. Vacuum removal of hydrogen out of a crystal lattice without change of symmetry is similar to thermoelectronic emission. Therefore we suggest the name of this new phenomenon as “hydrogen thermoemission“ from crystals. Thus hydrogen thermoemission is complete removal of hydrogen atoms out of crystal structure without changing the crystal symmetry. The aim of this chapter is generalizing the results of neutron diffraction investigations of low-temperature hydrogen thermoemission phenomenon from powder crystals of Ti-N-H, Zr-N-H, Ti-C-H, Ti-H systems and of rare earth metal trihydroxides R(OH)3, were R is La, Nd or Pr. We will also look into formation of regularities and structural features of hydrogen induced phases.

2. Experiment techniques

2.1 Choice of method of structural research

Neutron diffraction method was chosen as the basic method of structure study which allows to obtain the largest and reliable information in the structural analysis of the alloys consisting of components (metals and hydrogen) with strongly differing order numbers. Neutron diffraction patterns were obtained using the neutron diffractometer mounted at the

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thermal column of the nuclear reactor of the Institute of Nuclear Physics of Uzbekistan

Academy Sciences (=0,1085 nm) at room temperature. X-ray diffraction patterns were ob-tained using the X-ray diffractometer DRON – 3M with CuК – radiation ( = 0.15418 nm). 2.2 Calculation methods of crystal structure determination

The observed integral intensity Ik(calc) = Ihkl of diffraction reflections from planes with

Miller indexes h k l for a cylindrical sample is calculated using the formula (Bacon, 1975):

23 2hkl 0 hkl hklI I ( h / 2 r)(V / ')(p N F hkl /sin sin2 )A exp 2W │ , (1) where I0(3h/2r)(V/') = k is the constant dependent on geometry of the device and a sample; phkl - the multiplicity factor ; Fhkl2 - the structural factor for reflection (h k l); Ahkl - the factor of absorption (for the objects of our study the absorption factor is insignificant and, so, in calculations it was neglected), r - distance from a sample up to the counter; h - height of a crack of the counter; N - number of elementary cells in unit volume; V - volume of the

studied sample; and ' - experimental and calculated density of the studied sample; W - the thermal factor which is determined by equation:

2 2 2 2W (8 / 3)sin /u , (2) where 2u - atomic mean-square displacement. The structure factor F2 indicates the

functional dependence of reflection intensities upon atom positions in the unit cell. This

dependence is the function both of atom coordinates and Miller indexes of the reflex. The

general formula of structure factor is:

2 22

cos2 ( ) sin 2 ( )n n n n n n n nn n

F b hx ky lz b hx ky lz , (3) where xn, yn, zn are atom coordinates expressed in fractional units of the corresponding

parameter; bn - amplitude of neutron scattering on the atom nuclei. Summation is made over

all atoms of the unit cell. Determination of the structure was carried out by “trial-and-error”

method. The method consists in the next: the obtained experimental both angle positions

and intensity of diffraction maxima are compared with the values calculated theoretically

within the framework of the certain model. The most probable models of structure (taking

into account X-ray data) were selected, and for each model the refinement of atom

coordinates and position occupancy by least-square method of Rietveld (Rietveld, 1969) was

carried out. At that, the refined parameters could be varied both together and separately.

The model which yielded the best results (the minimal R-factor) was chosen. The structure

model is correct if the experimental positions of maxima coincide with calculated ones and

also if discrepancy indexes R are minimal. Last years for the refinement of crystal structures

using X-ray and neutron diffraction data, full-profile method offered by Rietveld is widely

used. The main point of this method is minimization of RP by means of full-profile

processing of powder neutron diffraction data. At that, over the Bragg’s reflections range

the discrepancy indexes are calculated using both the full profile (RP) and weights of each

point (RWP) and intensities of Bragg’s reflections (RBr). Now there is the software package for

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calculation and refinement of crystal structures on the basis of this method. In the present

work the software packages have been used DBW 3.2 (Young & Wilas, 1982).

2.3 Method of thermal treatment of samples

Powder samples with grain sizes of < 60 μm were studied. Dehydrogenation of hydro-genous compounds was carried out under persistent evacuation (at vacuum < 5.310-3 Pa). Thermal treatment of the samples under persistent evacuation was carried out in the vacu-

um furnace of SShVL–type. The principle scheme of this vacuum furnace is shown in Fig. 1.

For carrying out annealing the sample (1) was placed in enclosed volume (2) on a spe-cial

support (3). For vacuum control the vacuum-measuring block VIT-3 (4) and manometric

lamps PMT-2 (5) and PMI-3 (6) were used. By means of mechanical (7) and diffusion (8)

pumps in enclosed volume vacuum not worse than 5.310-3 Pа was made. After that, vacuum heat treatment of a sample began: temperature was slowly raised since the room

one up to necessary size with a step of 25 ºC; a sample was exposed at each temperature at

first during 24 and then during 36 and 48 hours. Temperature of a sample was controlled by

potentiometer (9) with help of PPR (platinum – platinum-rhodium) thermocouple (10).

Process was continued until the whole hydrogen leaves a sample. It should be noted that we

observed vacuum to be not worse. If vacuum became insignificantly worse, temperature

was decreased until vacuum was restored up to 5,310-3 Pа. After annealing at each temperature with certain exposure time (24, 36 and 48 h.) neutron diffraction pattern was

obtained. Hydrogen quantity in samples was determined as decline of incoherent

background caused by incoherent neutron scattering on H nuclei. Since hydrogen nucleus

has the largest amplitude of incoherent neutron scatting, its background scatting falls away

with increase of Bragg angle (Bacon, 1975). The hydrogen content was by chemical analysis

and controlled by the analysis of neutron structure data using the full-profile method of

analysis intensities of neutron patterns (Young & Wilas, 1982).

Fig. 1. The principle circuit of the high-temperature vacuum furnace of SShVL-type. 1-powder sample in metal cassette; 2 – enclosed volume; 3 - support for mounting of a sample; 4- vacuum-measuring block VIT-3; 5-manometric lamp PMT-2 for measuring forevacuum; 6-manometric lamp PMI-3 for measuring high vacuum; 7-mechanical pump; 8- diffusion pump; 9-potentiometer PP-63; 10-thermocouple PPR; 11-, 12-, 13-, 14 - vacuum gates.

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3. Results

3.1 Hydrogen thermoemission in the Zr-N-H system

The solid solution ZrN0.43H0.38 for studying was prepared by self-propagating high-

temperature synthesis. This method based on exothermal reaction of initial reagents. As

initial materials nitrogen of the "extra pure" brand and Zr powder of the M-41 brand were

taken. According to nameplate, the Zr powder contained 0.45 mas % of hydrogen. The

pressure of nitrogen in a constant pressure bomb was 7107 Pa. After synthesis the sample was exposed to homogenizing annealing in evacuated and sealed-off quartz ampoule at

temperature 1000 °C during 12 h. Then for fixation of the high-temperature state samples

were hardened in air. Powder samples with grain sizes no more than 60 μm were studied. Samples composition was determined by chemical analysis and was controlled by

minimizing the R factors of structure determination using neutron diffraction patterns.

Dehydrogenation of hydrogenous compounds was carried out under persistent evacuation

(at vacuum not more than 5.310-3 Pa) (Khidirov et al., 2009). After dehydrogenating at every temperature a neutron diffraction pattern was surveyed. Hydrogen quantity in

samples was watched as decline of incoherent background caused by incoherent neutron

scattering on H nuclei. Since hydrogen nucleus has the largest amplitude of incoherent

neutron scattering, its background scattering falls away with the increase of Bragg angle

(Bacon, 1975). The hydrogen content was also estimated by the analysis of reflection

intensities and sometimes by chemical analysis. As an initial sample the single-phase

ordered solid solution ZrN0.43H0,38 was taken. According to X-ray structure analysis, the

sample was single-phase and homogeneous and had hexagonal close-packed structure with

unit cell parameters: a = 0.3274 ± 0.0002; c = 0.5321 ± 0.0003 nm; c/ a =1.625. The neutron

diffraction pattern of the initial solid solution ZrN0,43H0,38 is shown in Fig. 2,a. Uniform slope

of incoherent background points to hydrogen presence. The neutron diffraction data of the

solid solution show that nitrogen atoms are ordered over one of two types of octahedral

interstices alternating along с axis, and hydrogen atoms − over tetrahedral interstices of hexagonal close-packed (HCP) metal structure; unit cell parameters are: a = 0.3274; c =

0.5321 nm (c/a = 1.625). The metal atoms (zZr=0.243) are displaced along c axis from their ideal

positions (zid=1/4) toward the plane of 1 (a) octahedrons occupied with nitrogen atoms (Table

1). So, the initial sample is the ordered solid solution of nitrogen and hydrogen in HCP

structure of -Zr: it is ' - Zr2N0.86H0.76 phase, space group P 3 m1 structure of anti-CdI2 type on nitrogen. Dehydrogenation of the '- Zr2N0.86H0.76 phase (of composition ZrN0.43H0.38) was carried out by a regime of step annealing within the temperature range 100 °C ≤T


159

Atom Position Atom coordinates

B, nm2 B, nm2 n n x y z z

Zr 2 (d) 1/3 2/3 0.243 0.001 0.0039 0.0005 2

N 1 (a) 0 0 0 0.0072 0.0009 0.86 0.05

H 2 (d) 1/3 2/3 0.619 0.002 0.0107 0.0042 0.76 0.04

Rp = 1.9; Rwp = 2.5; R Br = 4.4 %

Table 1. Structure characteristics and discrepancy indices R of the initial solid solution

ZrN0.43H0.38 in the space group P 3 m1 Notice: B – individual thermal factor; n - occupancy of

a position.

Fig. 2. Neutron diffraction patterns of solid solution: a – Zr2N0.86H0.76 ('; the space group 3 1Р m ); b - Zr2N0.86 ('; the space group 3 1Р m ); c - Zr2N0, 86 (''; the space group Pnnm); d -

ZrN0,43 ('3L ; the space group P63/mmc); e –mixture of the and δ phases.

annealed at 1000 °C for 3 h in evacuated and sealed quartz ampoule. As a result of annealing

the dehydrogenated sample disintegrated into disordered solid solution of nitrogen in -Zr − hexagonal phase (structure of L3 type) and a cubic -phase (Fig.2e) corresponding to the equilibrium phase diagram of Zr-N system (Gusev, 2001). It should be noted that the

annealing of hydrogenous Zr2N0,86H0,76 phase under similar conditions does not lead to decay

or change of crystal structure. Further annealing of the disintegrated sample (mixture of +δ phases) at 375 °C for 36 h did not lead to recovery of the phase with anti-CdI2 structure. As the

crystal structure of -Zr2N0,86 phase has been stabilized by hydrogen (then all hydrogen atoms are removed) and it is not observed in the phase diagram, this phase is offered to name

the hydrogen induced phase, unlike hydrogenous one. For the hydrogen induced phase -Zr2N0,86 the least value of divergence factor RBr=6.4 is obtained with nitrogen atoms

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arranged over 1(a) octahedral positions (space group P 3 m1) with the free metal parameter

zzr=0.255 diffe-ring from the ideal value zid =1/4 and from z=0.243 for corresponding

hydrogenous phase (Table 2). Hence, in the hydrogen induced phase metal atoms

displacement directions change: from the plane filled with nitrogen atoms toward the plane of

nitrogen vacancies (Fig. 3,a and 3,b). Hydrogen induced phase - Zr2N0,86 has the following lattice parameters: а =0.3264; с=0.5299 nm (с/a = 1.623). These values are less than those of corresponding hydrogenous phase. It should be noted that within the limits of experiment

errors the axes relation c/a is the same for the hydrogen induced phase and the hydrogenous

phase. As a result of hydrogen removal out of hexag-onal lattice its near isotropic compression

occurs. Analysis of neutron diffraction pattern of the phase Zr2N0.86H0.76 dehydrogenated at

450-650 °C (Fig. 2,c) shows that at these tempera-tures the ordered orthorhombic Zr2N0.86 phase with structure of anti-CaCl2 type ('' phase) is formed. The diffraction pattern of this phase has been indexed in orthorhombic system (space group Pnnm) with the lattice

parameters: a = 0.5632 nm 3 ao, b = 0.5252 nm co, c = 0.3256 nm ao where ao and co are lattice parameters of the initial hexagonal phase. The unit cell of this phase is shown in Fig. 3,c.

A good agreement between experimental and calculated intensities (R = 6.3 %) is achieved

given that four Zr atoms are in positions 4 (g), the nitrogen atoms occupy mainly positions 2

(a) and partially − positions 2 (c) (Table 3). At 650 °C the partial disordering of nitrogen atoms (~20 %), followed by their transition to octahedral interstices of another type − 1 (с), takes place. In neutron diagram of the sample, dehydroge-nated at 800 °C, has been reflections from

hexagonal phase with structure of '3L - type (α-phase) (Fig. 2, d). The unit cell of the α - phase is of anti - CaCl2 type (''- phase) is formed.

Atom Position Atom coordinates

B, nm2 B, nm2 n n x y z z

Zr 2 (d) 1/3 2/3 0.254 0.002 0.0043 0.0008 2

N 1 (a) 0 0 0 0.0032 0.0012 0.86 0.01

Rp = 2.9; Rwp = 3,9; R Br = 6.4 %

Table 2. Structure characteristics and discrepancy indices R of hydrogen induced phase α'-Zr2N0.86 (obtained at temperature 375 oC) in the space group P 3 m1.

The diffraction pattern of this phase has been indexed in orthorhombic system (the space

group Pnnm) with the lattice parameters: a = 0.5632 nm ao, b = 0.5252 nm co, c = 0.3256 nm ao where ao and co are lattice parameters of the initial hexagonal phase. The unit cell of this phase is shown in Fig.3, c. A good agreement between experimental and calculated

inten-sities (R = 6.3 %) is achieved given that four Zr atoms are in positions 4 (g), nitrogen

atoms occupy mainly positions 2 (a) and partially − positions 2 (c) (Table 3). At 650 oC the partial disordering of nitrogen atoms (~20 %), followed by their transition to octahedral

interstices of another type − 2 (с), takes place. In neutron diagram of the sample, dehydrogenated at 800 oC, has been reflections from hexagonal phase with structure of '3L -

type (α phase) (Fig. 2, d). The unit cell of the α-phase is shown in Fig. 3, d. We also investigated mutual transformation of the metastable phases. The annealing of the hydrogen induced phase at temperatures 450-650 oC results in formation of ''phase. In

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other words, in this temperature range phase transition from to '' is observed. The annealing of the '' phase at 400- 375 oC during 36 h does not lead to restoration of phase. Hence, 459, ,,C transition is monotropic (nonreversible) and phase can be induced only by hydrogen thermoemission at temperature below that the temperature of '' phase formation. By increasing the temperature up to 750 -800 oC, the '' phase passes into -phase (disordered oversaturated solid solution of nitrogen in a lattice of -Zr) with structure of '3L -type (Fig.2, d). This transition is enantiotropic (reversible) because the ''-phase is formed again by annealing the - ZrN0.43 phase at temperatures 650-450 oC. The annealing of the phases at 1000 oC results in their disintegration into to and phases (Fig. 2, e). No any annealing of samples that have been decayed into and δ phases does not lead to formation of , '' and phases in a single-phase kind. Therefore, the found phase transitions →''→ occur in metastable state. The formed metastable phases and phase transitions between them are schematically shown in Fig. 4. Thus, by means of dehydroge-nation of the

preliminary hydrogenated sample, three modifications of the oversaturated solid solution

ZrN0.43 which do not exist in the equilibrium phase diagram have been obtained.

Fig. 3. Unit cells of the hydrogenous solid solution ZrN0.86H0.76 and of phases initiated by low-temperature vacuum extraction of hydrogen: 1–metal atoms; 2–nitrogen atoms; 3–nitrogen vacancies; 4–hydrogen atoms; 5–hydrogen vacancies; 6–statistic arrangement of nitrogen atoms.

Atom PositionAtom coordinates

B, nm2 B, nm2 n n x x y y z

Zr 4 (g) 0.324 0.002 0.271 0.001 0 0.39 0.06 4

N 2 (a) 0 0 0 0.53 0.13 1.38 0.02

N 2 (c) 0 1/2 0 0.53 0.13 0.34 0.02

Rp = 4.5; Rwp = 6.0 ; RBr = 6.7 %

Table 3. Structure characteristics and discrepancy indices R of the ''-Zr2N0. 86 hydrogen induced phase in the space group 『nnm.

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Fig. 4. Scheme of obtaining and phase transformations of metastable oversaturated solid solution in Zr-N system.

In Fig. 4 these modifications are outlined by dashed lines (the small rectangle). Hydrogena-

tion of solid solution of the Zr-N system (Gusev, 2001) reveals that under a hydrogen

pressure of 『105 Pa, all samples, including biphase ones, form the sol. s. -Zr2N0.86H0.76 (space group P 3 m1). This inverse process also is shown in Fig. 4. In these cases, the

exposure time required for hydrogen saturation up to the concentration H/Zr = 0.38 will be

different at various temperatures. The scheme of reversible and nonreversible phase

transformations in metastable state of the ZrN0.43 is shown in Fig. 5.

Fig. 5. Scheme of phase transformations on the basis of the hydrogen induced phase - Zr2N0.86.

3.2 Hydrogen thermoemission in Ti-N(C)-H systems

Low-temperature vacuum evacuation of hydrogen has been investigated in the ordered

solid solution '-Ti2N0.52H0.30 (space group P 3 m1) and the disordered solid solution

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Fig. 6. Neutron diffraction patterns of the -Ti2N0.52H0.30 phase (a) and the -Ti2N0.52 hydrogen induced phase (b).

-TiN0.26H0.15 (space group P63/mmc) (Khidirov et al. 1991). In Fig.6, a, the neutron diffractogram of the ordered solid solution '- Ti2N0.52H0.30 is shown. A declining incoherent background is evident. After vacuum annealing at 370 0C this background disappears (Fig.

6,b). This provides evidence for the removal of hydrogen from the lattice, findings are

confirmed by chemical analysis. As in the neutron pattern (Fig. 6, b), all selective reflections

are the same. The ordered structure of the solid solution has not changed. In this way, the

"frozen" hydrogen-free metastable phase of Ti-N system − the hydrogen induced '-Ti2N0.52 phase is obtained. Processing the neutron data by Rietveld method has shown that the value

of the free metal parameter z changes in the formed hydrogen induced phase. It changes

from z = 0.237 – for the hydrogenous solid solution (that is less than z = ¼ for the ideal

position) to z = 0.260 – for the hydrogen induced phase (i.e. greater than z ideal). Annealing

the obtained phase in an evacuated and sealed quartz ampoule at T> 370 0C results in decay

corresponding to the equilibrium phase diagram of the Ti-N system (Gusev, 2001). Low-

temperature vacuum evacuation of hydrogen was also studied in solid soluti-ons

TiC0,45H0,90, TiC0,45H0,64, and TiC0.35H0.43 (Khidirov et al., 2008). In the neutron patterns of the

initial solid solutions (Fig. 7), superstructure reflections are visible together with structural

reflections and they are indexed within space group P 3 m1. According to neu-tron structure

analysis of the investigated solid solutions, thermoemission of H is virtu-ally not observed

up to 250 °C. In solid solutions with a large carbon concentration, the separation of pure

titanium and face-centered cubic phase (most probably carbohydride TiCxHy) was found

above 275°C. By further increasing the temperature, the quantity of precipitate phases

increases; up to 400°C, the incoherent background slope is kept constant. Thus, solid

solutions with high concentration of carbon (and hydrogen) − TiC0,45H0,90, TiC0,45H0,64, are thermostable in vacuum only at temperatures T < 275 °C, and in HCP and face-centered

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cubic (FCC) phases hydrogen remains up to 400°C. In the neutron diagram of the solid

solution TiC0,35H0,43 (-Ti2C0.70H0.86) after vacuum annealing at 275°C, there are the same reflections which have been observed in the neutron pattern of the initial hydrogenous

sample (Fig. 8); incoherent background, however, is completely absent.

Fig. 7. Neutron diffraction patterns of the -Ti2C0.70H0.86 - phase: solid line - calculated, dots – experimental, – differential one. Miller indices are indicated above the reflections.

Fig. 8. Neutron diffraction patterns of the ordered -Ti2C0.70 – phase. The notations are the same as in Fig. 7. Calculated diagram was obtained in sp. gr. P 3 m1 with zTi = 0.305

however, is completely absent.

Analysis of the diffraction compositions corresponds to the formula TiC0,35. The initial ordered structure, however, remains unaltered. Therefore, hydrogen thermoemission at

275°C results in the formation of hexagonal ordered -Ti2C0.70 - phase in a metastable state. Analysis of the diffraction pattern by the Rietveld method proved a lack of hydrogen and showed that the sample composition corresponds to the formula TiC0,35. The initial ordered structure, however, remains unaltered. Therefore, hydrogen thermoemission at 275°C

results in formation of the hexagonal ordered -Ti2C0.70 - phase in a metastable state. It is necessary to note the large errors in the determination of lattice parameters and the high value of divergence factors (RBr =13,6 %). Evidently, these results are caused by the distortion of peak forms and wide scatter of points in the neutron diffraction pattern; this is because of strong static distortions of the lattice. The large errors do not allow comparison of the lattice parameters of hydrogen induced phase Ti2C0.70 and the initial hydrogenous phase.

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In the hydrogen induced -Ti2C0.70 phase the sign of metal atoms displacement (zTi = 0,305) relative to the ideal position (zid = 1/4) changes in comparison with the hydrogenous -Ti2C0.70H0.86 phase (zTi = 0,237). If the value of zTi in the hydrogen induced phase Ti2C0.70 is

assumed to be the same as in the hydrogenous phase -Ti2C0.70H0.86, an essential growth of RBr is also observed (up to 30 % at zTi = 0.250 and 33 % at zTi=0.237). Thus, in the hydrogen-

induced -Ti2C0.70–phase, just as in the hydrogen induced -Ti2N0.52 phase (Khidirov et al., 1991), the sign of metal atoms displacement relative to their ideal position changes in comparison with corresponding hydrogen-containing phases. As a result of this research, it can be concluded that in solid solutions of the Ti-C(N)-H system, complete thermoemission of hydrogen with a conservation of initial symmetry (caused by hydrogen) is observed in strongly defective structures. Apparently, in strongly defective solid solutions the interaction forces between atoms are weakened, and the temperature of hydrogen evacuation (Tev) is below that of redistribution of interstitial atoms or of recrystallization (Trecr): Tev

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of neutrons by hydrogen nuclei (Bacon, 1975). Diffraction patterns of other trihydroxides were similar. All rare earth metals from trihydroxides R(OH)3 (R=La to Yb) having UCl3-type hexagonal structure (Portnoy, Timofeeva, 1986). The structure can be described in P63/m space group, where the rare earth element atoms R occupy 2(d) positions and (OH) complexes occupy 6(k) positions having free parameters x=0.29, y=0.38 (Schubert, 1971). The crystal structure of R (OH)3 phases was studied by X-ray diffraction. It was difficult to determine the hydrogen atom coordinates using the X-ray method, because the scattering amplitude of hydrogen atoms is much less than that for rare earth metals. That is why the coordinates of the spherical anions (OH)– were ascribed to the hydrogen atoms. It seemed reasonable to try to determine the coordinates of hydrogen atoms using neutron diffraction

because in case of neutrons the scattering amplitude of hydrogen nuclei (bH = 0.37410 -3 nm ) is comparable with that for rare earth metals (bLa = 0.8310 -3 nm, bPr = 0.4410 -3 nm, bNd= 0.7210 -3 nm) (Bacon, 1975). The coordinates of hydrogen atoms in rare earth metal trihydroxides R (OH)3 (R is La, Pr, Nd) were determined using the neutron diffraction method (Khidirov & Om, 1991). All diffraction peaks in the neutron diagrams could be indexed in the hexagonal unit cell with the lattice parameters which are given in Table 4.

Sample a (nm) c (nm) c/a

La(OH)3 0.6529 0.0006 0.3852 0.0003 0.5895 0.0003 Pr(OH)3 0.6458 0.0005 0.3771 0.0002 0.5837 0.0004 Nd(OH)3 0.6437 0.0005 0.3747 0.0002 0.5821 0.0004

Table 4. The lattice parameters of R(OH)3.

Atom Position x y z

2 R 2(d) 1/3 2/3 1/4

6 O 6(k) 0.30 0.002 0.385 0.002 1/4 6 H 6(k) 0.16 0.003 0.288 0.004 1/4

Table 5. Structure characteristics of R(OH)3 in the space group P63/m.

Sample Rp , % Rwp , % RBr , %

La(OH)3 0.74 0.93 6.59

Pr(OH)3 0.54 0.72 7.01

Nd(OH)3 0.88 1.03 7.32

Table 6. R-factors for R(OH)3 in the P63/m space group.

The lattice parameters are close to those obtained by other authors (Portnoy & Timofeeva, 1986)] except c of Nd(OH)3. A decrease in the ratio c/a with increasing metal atom number

corresponds to the reduction of atomic radius in line of the La PrNd. The least Rfactor was obtained for the model in the P63/m space group (Table 5). For the investigated

samples this model yielded, the following R factors are given in Table 6. The Rfactor values can be takes into account the high background level of the neutron diffraction patterns caused by incoherent scattering of the hydrogen nuclei and thermal diffusion

scattering. The distance between the oxygen and hydrogen atoms in the OH pairs equals (0.082 0.007) nm. A difference curve (observed minus calculated) is shown in Fig. 9, a. It should be noted that if the same coordinates are formally ascribed to the oxygen and

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hydrogen atoms the R factor increases considerably. For example, for La(OH)3 RBr=35.7 %. Thus, our results show that, though hydrogen and oxygen atoms occupy the same positions 6(k), their coordinates differ.The unit cell of R(OH)3 is shown in Fig. 10. Oxygen and hydrogen atoms form dumb-bells oriented as is the oxygen atom is attracted and the hydrogen atom is repelled by the nearest metal atoms.

Fig. 10. The unit cell of R(OH)3.

4.2 Neutron diffraction study of hydrogen thermoemission from crystals of tryhydroxides rare-earth metals R(OH)3

For rare-earth metal trihydrooxides R(OH)3 three characteristic temperatures are observed

for hydrogen extraction at continuously pumped out high vacuum.

1. The temperature of practically complete hydrogen evacuation from the lattice without change in symmetry of crystal - Тevac (140-150 0』), at which the crystal structure does not change, but is significantly deformed (Fig. 9, b ). Neutron diffraction calculation of samples after dehydrogenation at 130-150 0C by means of full-profile Rietveld analysis demonstrates no hydrogen in the sample, though the crystal structure is conserved (Fig. 9, b). Significant decrease of diffraction refraction intensities shown by neutron diffraction analysis after vacuum dehydrogenation can be explained, first of all, by lack of hydrogen atoms in lattice; secondly, by lower gross number of dehydrogenated samples. The latter is related to the fact that for acceleration of the process of hydrogen separation from powder R(OH)3 the quantity of samples was taken three times less than that taken for the original powders of R(OH)3. After hydrogen removal one can observe widening of half-width and distortion of diffraction peaks form, which is caused by pronounced deformations in crystal lattice after hydrogen removal. The best agreement between experimental and calculated intensities of neutron-diffraction reflections (Fig. 9, b) and minimal errors in structure determination (R) can be obtained only assumed that these compounds are “trioxides” containing R[O3]. It is worth of mentioning that the temperature of hydrogen evacuation from lattice in all

trioxides R(OH)3 within errors in temperature determination (Т= 12 C) is almost the same. This can be explained by the fact that rare-earth metals La, Pr and Nd have similar valence electron shells and does not differ in sizes of atoms, where as trihydrooxides R(OH)3 have isomorphic structure. The results of neutron diffraction analysis calculations Pr[O]3 within space group P63/m are presented in Table 7 (Khidirov, 2011). One should notice large errors in determination of lattice parameters and large value of RBr.. It is apparently caused by

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distortion of peaks forms due to strong statistical lattice deformations appearing after hydrogen removal. Formation of such a compound in R - O systems contradicts to the valence conservation principle. Oxygen is double-valence, and rare-earth metals can be either three or four-valence. But all of them in Rr(OH)3 are three-valence. It is obvious that “trioxides” R[O3] have broken bonds and unpaired electrons, likewise that of radicals, that is they have excessive negative charge: R3+[O3]6-. This material can be stable only at relatively low temperatures and in medium free of hydrogen. Indeed, annealing of R[O3] or dehydrogenation of R(OH)3 at temperatures of 150 0C longer that 24 hours leads to its amorphisation, which is pointed by lack of selective diffraction reflections in neutron

diffraction analysis, and formation of small diffuse reflection at angles of 2 = 28 – 38 degrees. Similar phenomenon takes place at even slight increase of temperature (up to 180 0C). Obtained “trioxide” R[O3] due to its valence instability “tends” to trap three more protons. This can be accomplished by trapping in lattice of three hydrogen ions at first instance. Therefore, metastable “trioxides” in atmosphere, apparently, interact with water molecules, and retransform step by step back in the trihydrooxide R(OH)3, by compensating the excessive negative charge by three hydrogen atoms. This is demonstrated by the neutron diffraction analysis calculations R[O3], taken in 30 days after obtaining neutron diffraction patters and by repeated formation of incoherent background on the neutron diffraction patterns. In other words, the neutron diffraction patterns of R[O3] samples after exposition in atmosphere at temperatures 285 - 290 K (temperature in the reactor hall in winter) within 30 days are both qualitatively and quantitatively become identical to the corresponding neutron diffraction analysis patterns for R(OH)3. Hence, one can conclude that in atmosphere the «trioxide» R[O3] «self-cures» until complete restoration of the trihydro-oxides of rare earth metals R(OH)3. (Obviously, the reaction rate depends on temperature). Since the hydrogen concentration on Earth’s surface is rather low, one can assume that R[O]3 captures hydrogen, mainly from, water vapor in the air, leaving oxygen molecules free:

63 2 2334R O powder metastable crystal 6H O 4R OH powder stable crystal 3O

Аtom Number Pozition X y z Pr 2 2(d) 2/3 1/3 1/4

O 6 6 (k) 0.376±0.002 0.461±0.002 1/4

а= 0.658 ±0.018 せす; с = 0.381 ±0.006 せす; R= 0.60 %; R = 0.81 %; R = 13,7%

Table 7. Structural characteristics and R-factors of the “trioxide” Pr[O3] the model in the 63/m space group.

Therefore, «trioxide» R[O3] in atmosphere can simultaneously be a hydrogen absorber and

oxygen generator. Separation (by low-temperature removal) of hydrogen from 4R(OH)3 can

again lead to restoration of its capabilities to be a simultaneous hydrogen accumulator and

oxygen generator in a medium containing water molecules. The cycle described above can

be accomplished many times. It is shown in Fig. 11.

2. Amorphisation temperature–Тamorph. (180 0』), at which the selective diffraction reflection disappear completely and diffuse reflection appear in the neutron diffraction pattern (Fig.

12). The experiments revealed that amorphous R[O3] in atmosphere crystallize spon-

taneously and transform into trihydrooxides of corresponding rare-earth metals in 1-1.5

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months at the temperature of the reactor’s hall in winter (15-20 0C). It was interesting to

study the kinetics of this process. We therefore periodically generated the neutron

diffraction patters of amorphous La[O3] (Fig. 13). During this process, the measured sample

was located continuously in the 6 mm in diameter vanadium cylinder with an open cap. Fig.

13 shows some peculiar neutron diffraction patterns taken within 50 days.

Fig. 11. The scheme of a cycle of reception «trioxides» R [O3] from R (OH)3 in continuously pumped out high vacuum and repeated formation R (OH) 3 from «try-oxides» R [O3] by self-curing in atmospheric conditions.

Fig. 12. The neutron diffraction pattern of the amorphous Pr[O3]

3. Recrystallization temperature – Тrecr. ( 2100』), at which cubic oxide phase of corres-ponding REM is formed (PrO2-x-phases instead of Pr(OH)3 or La2O3 – phases instead of La(OH)3 (Fig. 14). It is natural to assume that at such a temperature the lattice is abandoned not only by hydrogen atoms, but some part of oxygen atoms. In amorphous phase, at the very first stage, the nucleation centers of face-centered cubic oxide phase are formed (Fig. 13). Further, the oxide La2O3 – phase intensity increases and diffuse reflection intensities decrease demonstrating that the formed oxide phase grows instead of amorphous phase. It is interesting to observe that on the 5-th day after nucleation of the oxide phase, it doеs not appear in the neutron diffraction patterns. However, one can observe some small diffraction

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Fig. 13. Neutron diffraction patterns of amorphous La [O3], removed during spontaneous crystallization and its transformation in La (OH) 3: amorphous La [O3] (1); after its endurance in atmospheric conditions within 5 (2), 10 (3), 15 (4), 20 (5), 25 (6), 30 (7), 35 (8), 40 (9), 45 (10), 50 (11) days, accordingly.

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peaks of lanthanum trihydrooxide La(OH)3 (Fig. 13-5). These peaks grow in some time,

whereas the diffuse reflections decrease and finally disappear (Fig. 13-8, 13-9, 13-10). In the

beginning the diffraction peaks of trihydrooxide La(OH)3 are strongly distorted, but finally

they acquire ideal form. Thus, duration of processes of spontaneous crystallizationand

complete transformation of amorphous La[O3] into La(OH)3 in atmosphere at temperatures

15-20 0C is approximately 45-50 days. Certainly, the observed reaction rate depends on air

temperature and humidity. The discovered interesting phenomenon consists of spontaneous

crystallization of amorphous matter R[O3], formed by hydrogen thermoemission, and its

reverse transformation into trihydrooxide R(OH)3 in normal atmosphere conditions with

selective absorption of hydrogen from water molecules and gaseous environment, having

hydrogen partial pressure.

Fig. 14. Neutron diffraction pattern of the cubic PrO2-x phase.

5. Low-temperature vacuum evacuation of hydrogen out of crystal lattice of interstitial alloy TiH2

We have carried out a neutron diffraction study of the phenomenon of hydrogen thermo-

emission out of a lattice of the cubic phase of titanium dihydride TiH1.95 at temperatures 50 ÷

250°C in continuously pumped out vacuum not worse than ~ 5,3·10-3 Pa. An initial sample

TiH1.95 has been prepared by direct hydrogenation of a powder of metal by hydrogen at

pressure 105 Pa. The purity of initial titanium iodide was 99.98 %. For more uniform

distribution of hydrogen the samples after saturation were annealed in the pumped out and

sealed quartz ampoule during 48 h at temperature 120°C. As X-ray diffraction patterns have

shown, the obtained sample was single-phase and homogeneous on composition and had

face-centered cubic lattice with parameter a = 0.4454± 0.0001 nm. According to the work

(Kornilov, 1975), this lattice parameter corresponds to face-centered cubic phase of com-

pound TiH1,95 (δ-phase). Its processing by Rietveld full-profile analysis confirms the data of the work (Kornilov, 1975): the crystal structure of the cubic phase is described within the

framework of space group Fm3m. Dehydrogenating of TiH1.95 began from the room

temperature with a step of 25°C and with exposure time at each temperature not less than 24

h. After dehydrogenating neutron diffraction patterns were surveyed at each temperature,

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Fig. 15. Neutron diffraction patterns of the sample of titanium hydride TiH1.95: (a) – initial and (b) – after vacuum thermal treatment at 200 ºC. The notations are the same as in Fig. 7.

and content of hydrogen was controlled by neutron diffraction data using both

minimization of the R-factors and the relation of intensities of maxima with even (002) and

odd (111) Miller indices of the face-centered cubic phase. Up to temperature 100°C in

diffraction pattern neither qualitative, nor quantitative changes were not found. According

to neutron structure analysis, at evacuation of hydrogen in the temperature interval of 100 -

175°C the decrease of hydrogen concentration from H/Ti=1,84 up to 1,74 (± 0,04) is observed

at practically constant lattice parameter of the initial cubic phase (a = 0.4454±0.0001 nm for

TiH1.74). When evacuating hydrogen at temperature 200°C the splitting of some diffraction

reflections has been found out (Fig. 15). The analysis of character of splitting showed that

cubic lattice of dihydride has transformed to tetragonal. According to X-ray structure

analysis, the parameters of the new unit cell were a =0.3182 ± 0,0005; c = 0.4413 ± 0,0007 nm,

degree of tetragonal alteration is c/a = 1,386. The unit cell parameters of body-centered

tetragonal (BCT) phase relate to the initial (cubic) one by the relation: at ≈ ac/√2; ct ≈ cc. Similar structure was found for compositions TiH1.75-2.00 (Kornilov, 1975) and also was

described in (Bashkin et al., 1993) where it was considered in face-centered tetragonal (FCT)

aspect. The recalculation of the parameters received by us for face-centered tetragonal unit

cell gives: aFCT =0.4418; cFCT = 0.4472 nm; c/a ≈ 1.01. The crystal structure of the bode-centered tetragonal phase is described in the framework of space group I4/mmm. The structure

characteristic of the bode–centered tetragonal phase, determined from neutron diffraction

data and the divergence factors of structure determination are given in Table 8. Preservation

of intense incoherent background in the neutron diffraction pattern testifies that in the

lattice there was still a lot of hydrogen. If to increase temperature up to 225°C, it is observed

the significant reduction of incoherent background in the neutron pattern and also a decay

of quasistoichiometric face-centered cubic TiH1.95 phase onto nonstoichiometric the FCC

cubic TiH2-x phase and pure Ti. Thus, a change of lattice symmetry occurs before the

complete removal of hydrogen; in the TiH1.95 , recrystallization is lower than temperature of

evacuation: Trecryst < Tevac (at vacuum of P ~ 5,3·10-3 Pa). The obtained results allow one to

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conclude that vacuum 5,3x10-3 Pa (in which TiH2 dehydrogenating was carried out) is

insufficient for removal of hydrogen out of a lattice of metal hydride without a change of

crystal symmetry and for obtaining pure titanium with face-centered cubic lattice. For

achievement of the given purpose at the temperatures lower than the temperature of

recrystallization or decay it is necessary to create high vacuum (about ~ 10-9 ÷ 10-12 Pa).

AAtoms

Number of atoms

Positions に/a y/b z/c Beff., nm2

0 0 0

Ti 2 2 (a) 1/2 1/2 1/2

0 1/2 1/4

1/2 0 1/4 0.0041±

H 3.48± 0,04 4 (d) 1/2 0 3/4 0,0008

0 1/2 ¾

Rwp = 3,8 %; RBr = 3.7 %; RF =2.8 %

Table 8. Structure characteristics within the framework of space group I4/mmm and divergence factors of the bode-centered tetragonal phase of TiH1.74 obtained at the temperature 200°C under persistent evacuation.

6. Discussions

The formation of observed hydrogen induced phases and phase transitions in metastable state of Zr-N-H system may be explained as follows. Under 375ºC hydrogen atoms are removed out of the lattice but the configuration of relatively heavy atoms of the matrix does not change. As a result it is possible to obtain the "frozen" structure stabilized previously by hydrogen using thermoemission. At temperatures 450ºC ≤ T ≤ 650ºC the diffusive mobility of nitrogen atoms increases, making possible the ordering type change. Further temperature rise causes large displacements of atoms from lattice sites, leads to disorder in distribution of nitrogen atoms. The disordered state occurs typically for oversaturated solid solutions. This state precedes decay process starting above 800ºC. At temperatures below 700ºC decay process is strongly hindered, as it requires: nitrogen atoms to move at macroscopic distances and the power barrier to be overcome. It is necessary for formation of nucleus of new phase, namely, cubic zirconium nitride. However, ordering process needs only redistribution of nitrogen atoms and their movement at distances about interatomic space. As a result, during low-temperature annealing (400ºC

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Hydroge-nous phase

Sys-tem

Space group

Before hydrogen Evacuation

Тevac.,0』

After hydrogen Evacuation

a, nm c, nm c/a Hydr.

inducedphase

a, nm c, nm c/a

Ordered TiN0,26H0,15

HCP 3 1Р m 0.2978 0.4795 1.61 370 Ti2N0,52 0.2956 0.4765 1.61

Disordered TiN0,26H0,15

HCP Р63/mmc 0.2978 0.4795 1.61 320 TiN0,26 0.2956 0.4765 1.61

ZrN0.43H0,38 HCP 3 1Р m 0.3276 0.5324 1.63 400 ZrN0,38 0.3266 0.5299 1.63

ZrN0,27H0,35 HCPHCP

3 1Р m 0.3274 0.5240 1.60350 ZrN0,27

0.3266 0.5226 1.60

3 1Р m 0.3274 0.5332 1.62 0.3270 0.5299 1.62 TiC0,38H0,43 HCP 3 1Р m 0.3024 0.4894 1.62 275 Ti2C0,76 0.3020 0.4990 1.62

Pr(OH)3 HCP P3/m 0.6458 0.3771 0.58 150 Pr[O3] 0.6580 0.381 0.58 TiH1,95 FCC Fm3m 0.4454 100 TiH1,85 0.4454

TiH1,95 FCC Fm3m 0.4454 170 TiH1,75 0.4454

Table 9. Some structure characteristics of solid solutions before and after hydrogen thermoemission.

effects may be explained by various characters of chemical bond of nitrogen and hydrogen

with metal atoms. Hydrogen atoms create isotropic field of elastic stresses resulting in

isotropic extension of the structure. One may think that after removing hydrogen atoms

isotropic compression of a lattice takes place. According to (Grigorovich & Sheftel’, 1980),

chemical bonds in Me-N systems are realized so that nitrogen atoms send a part of valence

electrons to the conductivity zone of the metal, and thus nitrogen and metal atoms own

common valence electrons. It is obvious that positively charged Ме and N ions repel each other. According to (Geld, 1985), hydrogen atoms are donors as compared to metal atoms. If

this is still rue in presence of nitrogen, metal atoms become negatively charged and will be

attracted to nitrogen atoms. Hence the direction of displacement for metal atoms in

hydrogenous phase will be the opposite to that of metal atoms in hydrogen induced phase.

Hydrogen thermoemission in crystals can be explained as relief of potential wells in crystals

(Fig. 16). Heavy atoms and hydrogen atoms have different initial wells. Hence to overcome

the potential barrier less energy is required for hydrogen compared to heavier atoms.

Fig. 16. The relief of potential wells in solid solutions MeC(N)xHy (a) and in rare earth metal trihydroxides R(OH)3 (b).

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7. Conclusions

1. It is found a new phenomenon – complete thermoemission of hydrogen out of a H-

containing ground crystals which proceeds without altering of crystal symmetry; it is

proposed that these artificially-forming metastable phases be named by hydrogen induced

phases.

2. At low-temperature (Т = 375 oC) high-vacuum evacuations of hydrogen out of lattice of the solid solution ZrN0.43H0.38 the new, hydrogen-induced - Zr2N0.86 phase was obtained that is absent in the phase diagram of Zr-N system; it is impossible to obtain it by traditional

methods.

3. During formation of hydrogen induced phase two observations are made: isotropic

compression of the crystal lattice and change for metal atoms displacement directions.

Apparently, such effects may be explained by various characters of chemical bond of

nitrogen and hydrogen with metal atoms.

4. At rise in temperature of H evacuation two more modifications of the same compound

(Zr2N0.86 ) were obtained but they have somewhat different structure: at Т = 450 - 650 oC it is the orthorhombic ordered '' phase (sp. gr. Pnnm); at Т = 750 - 800 oC - disordered hexagonal phase (sp. gr. P63/mmc). All the three modifications: , '' and are genetically connected to the initial phase but do not contain hydrogen. Between the found

modifications there are reversible and nonreversible phase transitions; conditions and

schemes of the transitions are presented.

5. It is established that it is possible to produce the unique metastable hydrogen induced

phases, which do not contain hydrogen but keep the structure of initial phase as "frozen", by

means of evacuation of hydrogen out of a lattice of some solid solutions of the Zr(Ti) - N - H

system using special regimes of vacuum heat treatment. It shows additional opportunities of

obtaining new compounds with defined service characteristics by hydrogenating and

dehydrogenating.

6. It has been studied the influence of vacuum thermal treatment on stability of the structure

of the solid solutions TiC0,45H0,90, TiC0,45H0,64 and TiC0.35H0.43. It is shown that the

phenomenon of complete thermoemission of hydrogen is observed in TiC0.35H0.43 at

temperature 275˚C under persistent evacuation not worse than 5.3x10-3 Pa. In other solid

solutions (with the large concentrations of carbon and hydrogen), starting since the same

temperature, the decay of the solid solution is observed, α -Ti and FCC titanium carbohydride TiCxHy being formed. The conclusion is made that in solid solutions TiCxHy

thermoemission of H is observed only in strongly defective structures in which interaction

forces between atoms are weak and also temperature of hydrogen evacuation Tevac is lower

than temperature of interstitial atoms redistribution or of recrystallization Trecryst: Tevac <

Trecryst.

7. The crystal structure of metastable hydrogen induced Ti2C0.70 phase is strongly distorted

what is appeared in large scatter of points in neutron diagram and also in large errors of

determination of lattice parameters and great value of divergence factor RBr.. It is found the

effect of changing of direction of matrix atoms displacement relatively to their ideal position

after complete thermoemission of hydrogen out of lattice of the solid solution TiC0.35H0.43; it

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may be explained by change of character of interaction between the atoms after "soft"

removal of hydrogen.

8. It is shown that at evacuation of hydrogen out of the lattice of titanium hydride TiH1.95 in

the temperature interval of 100 - 175°C in continuously pumped out vacuum the decrease of

hydrogen concentration from H/Ti=1,84 up to 1,74 (± 0,04) is observed at practically

constant lattice parameter of the initial cubic phase (sp.gr.Fm3m). So, it is shown the

possibility of “soft” regulation of composition of titanium hydride without change of lattice

parameter. At temperature 200°C the body-centered tetragonal (BCT) phase is formed with

the unit cell parameters a = 0.3182 ± 0,0005; с = 0.4413 ± 0,o007 nm (space group I4/mmm); starting from the temperature of 225°C it is observed the significant reduction in hydrogen

content and also a decay of stoichiometric FCC TiH1.95 - phase onto nonstoichiometric FCC

TiH2-x phase and pure Ti. So, a change of symmetry of the lattice is occurred before complete

removal of hydrogen out of lattice of TiH1.95, thereby in TiH2 temperature of recrystallization

is lower than temperature of evacuation: Trecryst < Tevac at pressure of 5,3·10-3 Pa and such

vacuum is insufficient for obtaining pure titanium with FCC lattice by means of

dehydrogenation.

9. The obtained results on study of the phenomenon of hydrogen thermoemission in the

solid solutions TiC0,45H0,90, TiC0,45H0,64 and TiH1.95 allow one to conclude that vacuum 5.3·10-3

Pa (in which their dehydrogenating was carried out) is insufficient for removal of hydrogen

out of lattice of these phases without a change of crystal symmetry, and for achievement of

the given purpose at the temperatures, lower than the temperature of recrystallization or

decay, the higher vacuum is necessary.

10. By neutron diffraction is found that by low-temperature (Тэвакуаぬии = 130-150 0C ) removal of hydrogen (by thermoemission) from rare earth metal trihydrooxide R(OH)3

(were R is La, Pr or Nd) under continuous high vacuum evacuating, makes possible to

obtain metastable “trioxide” R[O]3 of radical type. Existence of such substance contradicts to

the valence law (oxygen is bivalent and Pr is trivalent in hydroxides). Such "trioxide" have a

superfluous negative charge: Pr3+[O3]6-. So they aspire "to capture" three more protons

(hydrogen ions) from a water molecule. Obviously, this substance can be stable at low

temperatures and in the mediums, which are not containing hydrogen. In the air at room

temperature this substance, most likely, interacting with water molecules, gradually again

turns into trihydroxide Pr(OH)3, compensating the superfluous negative charge by three

hydrogen atoms: . From this it follows that substance Pr[O3] can simultaneously be an

absorber of hydrogen and generator of oxygen at atmospheric conditions and in any

mediums which contains water molecules, without any prior processing like heating or high

pressure:

2 2334Pr O metastable powder crystal 6H O 4Pr OH stable powder crystal 3O Thus, the obtained material, without any prior processing like heating or high pressure, can

be simultaneously oxygen generator and hydrogen accumulator in any mediums.

Characteristics of Pr[O3] to transform into stable form Pr(OH)3 by selective bonding of

hydrogen from the hydrogen-containing environment allows implication of Pr[O3] as the

hydrogen selective absorber. The cycle described can be accomplished many times.

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11. Hydrogen thermoemission is a new method of production for new metastable crystal

phases of inorganic compounds which are not observed in phase diagrams and cannot be

obtained by conventional methods (quenching, annealing, irradiation, chemical reactions,

etc.). Hydrogen is used as a factor of the external system. Comparison of structural

peculiarities of hydrogen-induced and corresponding hydrogenous phases opens new sides

in understanding of crystal-chemistry of these compounds and crystal-chemical behavior of

hydrogen. With such a great variety of hydrogenous inorganic compounds, the clear

possibility exists to discover many new metastable crystal phases and materials with new

properties that have yet to be explored.

8. References

Bacon, G. E. (1975). Neutron Diffraction. Oxford: Clarendon Press, 3rd ed. England. Bashkin, I. O., Djyueva, l, M., Lityagina, L. M., Malyshev B. Yu. (1993). Concentration

dependence of lattice parameter of TiH2. Fizika tverdogo tela. Vol. 35, No 11. (December 1993), PP. 3104-3114, ISSN 0042-1294.

Geld P. V. & Ryabov P. A. (1985). Mokhracheva L. P. Hydrogen and physical properties of metals and alloys. Moscow: Metallurgiya, Russia.

Gusev, A. I. (2001). Order-disorder transformations and phase equilibrium in strongly nonstoichiometric compounds. Uspekhi fizicheskikh nauk. Vol. 170, No 1. (January 2001), pp. 3-40, ISSN 0042-1294.

Grigorovich, V. K. & Sheftel’, E. N. (1980). Dispersion hardening of refractory metals. Moscow: Nauka, Russia.

Khidirov, I. (2011). Study of the phenomenon of hydrogen thermoemission from crystal lattice of the trihydroxide Pr(OH)3 by neutron diffraction. International Scientific Journal for Alternative Energy and Ecology, No 5, (June 2011), pp. 19-22, ISSN 1608-8298.

Khidirov, I., Kurbanov, I. I. & Makhmudov, A. Sh. (1991). Hydrogen induced metastable phase in the Ti-N system. Metallofizika, Vol. 13, No 6, (June 1991), pp.43-47, ISSN 1024-1809.

Khidirov I., Mirzaev, B. B., Mukhtarova N. N., Bagolepov V. A., Savenko A. F., Pishuk V. K. (2008). Influence of vacuum thermal treatment on structure of solid solutioins TiCxHy. In the Book Carbon Nanomaterials in Clean Energy hydrogen Systems. Pp. 679- 686, ISBN 978-1-4020-8896, Dordrecht: Springer, The Netherlands.

Khidirov, I. &, Om, V. T. (1993). Localization of hydrogen Atoms in Rare Earth Metal Trihydroxides R(OH)3. Physica Status Solidi (a), Vol. 140, (September 1993), pp. K59-K62, ISSN 1862-6300.

Khidirov, I., Veziroglu, T. N. & Veziroglu A. (2009). Phase transitions in oversaturated solid

solution of nitrogen in the -Zr obtained by using hydrogen termoemission. International Scientific Journal for Alternative Energy and Ecology, No 2, (February 2009), pp. 8-14, ISSN 1608-8298.

Kornilov, I. I.(1975). Titan. Moscow: Nauka, Russia. Portnoy, K. B. & Timofeeva, N. I. (1986). Oxygen connection of rare-earth metals. Moscow:

Metallurgiya. Russia.

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Rietvild, H. M. (1969). Profile Refinement method for nuclear and magnetic structures.Journal of Applied Crystallography,Vol. 2, part 2, (June 1969), pp. 65-71, ISSN 0021-8898.

Schubert, K. (1971). Crystal structures of two-componental phases. Moscow: Metallurgiya, translation from the German language, Russia.

Young, R. A., Wilas, D. B. (1982). Profile Scope Functions in Rietveld Refinements. Journal of Applied Crystallography,Vol. 2, (February 1982), pp. 65-71, ISSN 0021-8898.

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Neutron DiffractionEdited by Prof. Irisali Khidirov

ISBN 978-953-51-0307-3Hard cover, 286 pagesPublisher InTechPublished online 14, March, 2012Published in print edition March, 2012

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Now neutron diffraction is widely applied for the research of crystal, magnetic structure and internal stress ofcrystalline materials of various classes, including nanocrystalls. In the present book, we make practically shortexcursion to modern state of neutron diffraction researches of crystal materials of various classes. The bookcontains a helpful information on a modern state of neutron diffraction researches of crystals for the broadspecialists interested in studying crystals and purposeful regulation of their service characteristics, since thecrystal structure, basically, defines their physical and mechanical properties. Some chapters of the book havemethodical character that can be useful to scientists, interested in possibilities of neutron diffraction. We hope,that results of last years presented in the book, can be a push to new ideas in studying of crystalline, magneticstructure and a macrostructure of usual crystal materials and nanocrystals. In turn, it can promote working outof new materials with new improved service characteristics and to origin of innovative ideas.

How to referenceIn order to correctly reference this scholarly work, feel free to copy and paste the following:

I. Khidirov (2012). Neutron Diffraction Study of Hydrogen Thermoemission Phenomenon from PowderCrystals, Neutron Diffraction, Prof. Irisali Khidirov (Ed.), ISBN: 978-953-51-0307-3, InTech, Available from:http://www.intechopen.com/books/neutron-diffraction/neutron-diffraction-study-of-hydrogen-thermoemission-phenomenon-from-powder-crystals

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